1. Field of the Invention
This invention relates to a developer for electrostatic images. More particularly, this invention relates to an electrostatic image developer which exhibits properties such as charge stability and fluidity at improved levels.
2. Prior Art
The electrophotography produces a visible image by forming an electrostatic image on the surface of a photoconductive element and exposing the surface bearing the electrostatic image to a developer which contains a toner charged to a polarity opposite to that of the image thereby inducing deposition of the toner on the image. The visible toner image consequently obtained, when necessary, may be transferred to a transfer paper and fixed thereon to obtain a copy.
The charging of the toner which is used for developing such electrostatic image is fulfilled in the two-component developing method by mixing and diffusing the toner with a substance generally called a carrier thereby imparting an electric charge to the toner, or in the one-component developing method by causing the toner to contact a developing sleeve or a toner regulating plate thereby imparting an electric charge to the toner. In any event, the steps of development and transfer of the image pose various problems unless the electric charge is imparted uniformly and stably to the toner. For example, the inferiority of developed image due to the lack of the developer soavenging effect and the developing electrode effect and the liability of the sensitizing surface to gather smear due to insufficient transfer of developed image are counted among the problems.
For the purpose of solving these problems, JP-A-52-135,739 proposes an electrostatic image developing toner which has as a component thereof a metal oxide or silicone dioxide powder treated with amino silane. Since this toner is capable of maintaining a positive charge property, it is enabled to exhibit its developing property and transferring property at improved levels. When the metal oxide powder or silicon dioxide powder is blended with the binding resin of the toner and the coloring agent such as carbon black, however, it inevitably poses problems such as consuming much time for dispersion and suffering ready agglomeration of the produced toner particles. For the purpose of improving the fluidity of toner and enhancing the stability of charging, the practice of adding hydrophobic colloidal silica to toner particles has been introduced to the art and put to use (as disclosed in JP-B-63-36,499, for example). The addition of such colloidal silica indeed results in improving the fluidity of toner. It nevertheless consumes much time for dispersion of components being mixed during the manufacture of toner as in the former case and fails to enhance the charging property of toner to a fully satisfactory extent. Specifically, it entails the problem that unduly heavy fogging and unduly low image density ensue when the total number of produced copies exceeds 10,000 to 20,000. This particular problem gains in consipicuity with rising temperature and moisture of the ambience.
JP-A-63-101,854 discloses a toner containing a polymethylsilsesquioxane powder possessing a negative charge property and JP-A-63-101,855 a toner containing a polymethylsilsesquioxane powder endowed with a positive charge property by a surface treatment with an amino group-containing silane compound. Though these toners enjoy fully satisfactory charge properties, they have the problem that they suffer loss of fluidity, tend to smear the sensitizing surface, and exhibit a poor transferring property because the polymethylsilsesquioxane has unduly low mechanical strength.